A wear phenomenon (hereinafter referred to fretting) known as micromotion wear in general occurs in various mechanical elements such as elements aimed at restricting the relative motion, e.g., shaft engaging parts, bolt joint parts, rivet joint parts, and tapered couplings, or elements accompanying minute reciprocation, e.g., rolling bearings, sliding bearings, ball bushings, spline shafts, flexible shaft couplings, universal joints, constant velocity joints, leaf springs, coil springs, electric contacts, valves with valve seats, and wire ropes. When transporting cars in particular, long-distant transportation is carried out by trailers and freight trains, whereby minute vibrations during the transportation cause fretting in bearing transfer surfaces, which is problematic.
Therefore, various methods have been under consideration in order to keep such a phenomenon from happening. One of such methods proposed chooses an appropriate lubricant, so as to prevent fretting from occurring.
Meanwhile, though a fretting prevention method using grease as a lubricant has been reported, the fretting resistance of greases has not fully been elucidated yet. For example, there are cases where greases in which the same thickener is compounded yield results contrary to each other concerning the fretting resistance depending on test methods. Also, though there have been many reports stating that additives containing phosphorus compounds such as phosphates and phosphate esters are preferred, their fretting resistance properties vary greatly depending on structures of the phosphorus compounds.
On the other hand, the inventors have found that a grease containing a compound (so-called urea type thickener) selected from the group consisting of urea-urethane compounds and urethane compounds, and a compound selected from the group consisting of paraffin oxides, diphenyl hydrogen phosphite, and hexamethylphosphoric triamide is excellent in the resistance to fretting, and disclosed this finding in Japanese Patent Application Laid-Open No. HEI 02-232297.